A battery generally includes a positive electrode (which is a cathode during discharge), a negative electrode (which is an anode during discharge) and an electrolyte for ion transport between the electrodes. Because of its low electrochemical oxidation/reduction potential and light weight, lithium (Li) is commonly used in both primary and rechargeable battery systems. Rechargeable batteries based Li-ion polymer cells are commercially available and widely used.
Li-ion battery systems generally include large amounts of liquid organic solvents in the electrolyte. The use of liquid organic solvents has associated safety concerns arising from their high degree of volatility and flammability, particularly in large format batteries. In addition, in the event of damage or corrosion of a battery, liquid electrolytes can leak and spread, causing further damage to other battery components or nearby equipment. In contrast, Li-ion polymer, or Li polymer batteries include a solid-polymer electrolyte rather than an organic solvent electrolyte, which mitigates many of the safety concerns associated with Li-ion batteries.
Both Li-ion and Li-ion polymer batteries can employ a graphite/Li intercalated material as the anode material. This limits the energy density of these batteries because the intercalated anode has only about 10% of the capacity of Li metal itself (˜370 mAh/g vs. ˜3800 mAh/g, respectively). Li metal cannot practically be used in batteries in combination with an organic electrolyte, due to safety concerns with regards to dendrite formation and the reactivity of Li metal with the organic solvent. As described in Applications of Electroactive Polymers, B. Scrosati (ed), Chapman and Hall (1993) and J. B. Bates et al., Solid State Ionics, 135 (2000) 33-45, these issues can be resolved by using a solid electrolyte (e.g., a ceramic such as LiPON, or a high molecular weight polyethylene oxide (PEO) doped with Li salt) of greater mechanical strength than liquid or gel-polymer systems. However, such solid electrolytes often have significantly lower ionic conductivity at room temperature than the alternatives, limiting their use to, e.g., high temperature batteries, thin film cells or other specialized applications.
Higher conductivities using solid polymer electrolytes may be achieved through the introduction of small molecule “plasticizers,” such as organic solvents or ionic liquids. However, safety issues, similar to those described above in connection with Li-ion batteries, arising from flammability or mechanical instability, may result from the use of such solvents or liquids. High molecular weight PEO-containing electrolytes may be modified to form a network using processes such as UV-initiated radical crosslinking and electron beam crosslinking, whereby these processes introduce bonds between polymer chains along the length of the chain to create an electrolyte of greater mechanical strength. See, for example, Kim et al., J. Power Sources, 195 (2010) 6130-6137. However, these electrolytes exhibit limited ionic conductivities that may be attributed, at least in part, to the random nature of the crosslinking reaction. In addition, residual photoinitiator species (for example, benzophenone) contained in such crosslinked electrolytes may be undesirable for long-term stability and electrochemical performance. FIG. 1A is a diagram showing how non-functionalized (i.e., non-telechelic) polymer 102 is subject to UV-initiated or electron-beam cross-linking 104 to essentially random crosslinking 106.